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Author version: Environ. Monit. Assess., vol.185(2); 2013; 1237-1253

Impact of eutrophication on the occurrence of Trichodesmium in the Cochin backwaters, the largest estuary along the west coast of India

G. D. Martin1, 2, *R. Jyothibabu1, N.V. Madhu1, K. K. Balachandran1, Maheswari Nair1, K.R. Muraleedharan1, P. K.

Arun1, C.K. Haridevi1, C. Revichandran1

1 CSIR National Institute of Oceanography, Regional Centre, Kochi - 682018, India

2 Cochin University of Science and Technology, Kochi - 682016, India *Corresponding author, E mail: rjyothibabu@nio.org; Phone: +91 484 2390814; Fax: +91 484 2390618

Abstract

Phytoplankton studies in early 1970’s have shown the annual dominance of diatoms and a seasonal abundance

of Trichodesmium in the lower reaches of the Cochin backwaters (CBW) and adjacent coastal Arabian Sea during

the pre-summer monsoon period (February to May). Surprisingly, more recent literature shows a complete

absence of Trichodesmium in the CBW after 1975 even though their seasonal occurrence in the adjacent coastal

Arabian Sea continued without much change. In order to understand this important ecological feature, we

analyzed the long term water quality data (1965 – 2005) from the lower reaches of the CBW. The analyses have

shown that salinity did not undergo any major change in the lower reaches over the years and values remained

>30 throughout the period. In contrast, a tremendous increase was well marked in levels of nitrate (NO3) and

phosphate (PO4) in the CBW after 1975 (av. 15 μM and 3.5 μM, respectively) compared to the period before (av.

2 μM and 0.9 μM respectively). Monthly time series data collected in 2004-2005 period from the lower reaches of

the CBW and coastal Arabian Sea have clearly shown that the physical characteristics like salinity, temperature,

water column stability and transparency in both regions are very similar during the pre-summer monsoon period.

In contrast, the nutrients level in the CBW is several folds higher (NO3 – 8 μM, PO4 – 4 μM, SiO4 – 10 μM and NH4

–19 μM) than the adjacent coastal Arabian Sea (NO3 - 0.7 μM, PO4 - 0.5 μM, SiO4 – 0.9 μM and NH4 - 0.6 μM).

The historic and fresh time series data evidences a close coupling between enriched levels of nutrients and the

absence of Trichodesmium in the Cochin backwaters

Key words: Eutrophication, Trichodesmium, Nutrients, Phytoplankton, Arabian Sea, Cochin backwaters

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1. Introduction

One of the well-documented consequences of human alterations of environment is the eutrophication of

estuaries and coastal seas. The term ‘eutrophication’ refers primarily to the increase in compounds of nitrogen

and phosphorus in aquatic ecosystems. Eutrophication may cause exponential growth of a few species of

phytoplankton causing the loss of biodiversity (Verlekar et al. 2006). High growth of a few species of

phytoplankton may disrupt the balance of the ecosystem by depletion of oxygen near the bottom, production of

toxins etc, thereby negatively influencing the associated organisms. Normally, eutrophication is a very gradual

natural process, but large-scale human activities can greatly accelerate the rate at which nutrients enter into

aquatic ecosystems (Anderson et al. 2002).

The deterioration of water quality of many Indian estuaries in recent times has been reported

(Balachandran et al. 2005; Mukhopadhyay et al. 2006; Ramaiah et al. 2006; Martin et al. 2011). The need for

protecting these estuaries from further eutrophication is an important concern due to mounting human settlements

and developmental activities. In this direction, Cochin backwaters, situated along the southwest coast of India, is

a good example to examine. It is the largest estuarine system along the southwest coast of India with high levels

of nutrients throughout the year (NO3 >8 �M, PO4 >3 �M, SiO4 >5 �M). The nutrients in backwater reach

exceptionally high levels (NO3 >50 �M, PO4 >50 �M, SiO4 >125 �M) during the summer monsoon (Saraladevi et

al. 1983; 1991; Sankaranarayanan et al. 1986; Jyothibabu et al. 2006; Martin et al. 2008, 2010). Runoff from

agriculture, industrial discharge, aquaculture and domestic wastes are the major sources of nutrients in the

backwater (Saraladevi et al. 1991; Vijayan et al. 1976; Madhu et al. 2007). The coastal region of the southeastern

Arabian Sea (also known as Laccadive Sea), which receive the influx of backwater, shows marked decrease in

nutrient and phytoplankton pigment levels due to mixing with marine waters (Sankaranarayanan and Qasim

1969; Saraladevi et al. 1991; Lierheimer and Banse 2002).

Trichodesmium is a gaseous nitrogen fixing cyanobacterium common in the tropical and temperate

waters (Capone et al. 1998; Devassy et al. 1978; Jyothibabu et al. 2003; Krishnan et al. 2007; Hegde et al. 2008).

Among several species of Trichodesmium potent to form red tides, two species Trichodesmium erythraeum and

T. thiebautii are common in Indian waters (Devassy et al. 1978; Nair et al. 1992; Krishnan et al. 2007; Jyothibabu

et al. 2008). The blooming of Trichodesmium in tropical waters is believed as a response to surface layer

stratification and nitrogen limitation (Carpenter et al. 1999; Krishnan et al. 2007; Hegde et al. 2008; Jyothibabu et

al. 2008). Along the Indian coasts, Trichodesmium blooms are common during the pre-summer monsoon period

(Devassy et al. 1978; Jyothibabu et al. 2003, 2008; Hegde et al. 2008) and its formation was reported in a wide

range of salinity (28-34) (Devassy et al. 1978; Nair et al. 1992; Rao and Sarojini 1992; Devassy and Goes 1988;

Jyothibabu et al. 2003, 2008; Krishnan et al. 2007).

Several records in the early 1970s have shown the occurrence of Trichodesmium in the lower reaches of

the Cochin backwaters during the pre-summer monsoon period (Gopinathan 1972; Gopinathan et al. 1974; also

see the review by Verma and Agarwal 2000). However, Trichodesmium has not been encountered in the Cochin

backwaters in any of the phytoplankton studies conducted in the last three decades (Table1). At the same time,

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the incidence of Trichodesmium bloom had frequently been reported from the adjacent coastal waters during the

pre-summer monsoon period (Nair et al. 1992; Krishnan et al. 2007; Ashadevi et al. 2010). Therefore, the

absence of Trichodesmium in the Cochin backwaters in recent decades is intriguing given that the physical

characteristics of the lower reaches of the backwaters and the coastal Arabian Sea are very similar during the

pre-summer monsoon period (Madhupratap 1987; Menon et al. 2000).

In order to study the possible reasons for the intriguing absence of Trichodesmium in the backwaters in

recent decades, we analyzed three sets of data (a) long term data of water quality in the lower reaches of CBW

and (b) the seasonal hydrographic conditions in the Cochin backwaters and coastal marine waters and (c)

monthly time series water quality data from the lower reaches of the CBW and coastal Arabian Sea spanning

from summer monsoon to spring intermonsoon (till the bloom occurred in the coastal Arabian Sea). This

examination is primarily because major physical factors like temperature, salinity and transparency are the

important on the occurrence and blooming of Trichodesmium (Capone et al. 1998; Hood et al. 2001). The major

objectives of the present study can be stated as (a) to understand the major environmental change that occurred

in the Cochin backwaters in 1970’s and to understand whether this change has any link to the absence of

Trichodesmium in the backwaters and (b) to delineate the reason behind the current absence of Trichodesmium

in the lower reaches of the backwaters and its presence in the adjacent coastal Arabian Sea.

2. Materials and methods

2.1. Study area

The estuarine system located around the city of Cochin (renamed as Kochi) is known as Cochin

backwaters. It consists of the northern part of the backwaters of Kerala which extends from Aleppey to Azhikode

(between Lat. 9o 30’ to 10o 10’N and Lon. 76o 15’ to 76o 25 ’E). The backwaters is a complex, shallow estuarine

network running parallel to the coastline of Kerala with two permanent opening to the Arabian Sea – one at

Cochin and the other at Azhikode. Six rivers (Pamba, Achancovil, Manimala, Meenachil, Periyar, and

Muvattupuzha) with their tributaries and several canals bring large volumes of freshwater into the backwaters.

Among these rivers, Periyar and Muvattupuzha discharge into the northern part of the backwaters and hence

have an active influence on the prevailing salinity in the Cochin backwaters.

Based on the climatology of the study area, seasons have traditionally been classified into

monsoon/summer monsoon/southwest monsoon (June to September), post-monsoon (October to January) and

pre-summer monsoon (February to May – see Menon et al. 2000). Among these seasons, summer monsoon

period accounts for 60-65% of the total annual rainfall in the study area (Menon et al. 2000). As a result of heavy

rainfall during the peak monsoon period, salinity over a large extent of the backwaters reaches near zero values.

During the post-summer monsoon period, river discharge into the backwaters diminishes and salinity gradually

increases. As pre - summer monsoon begins; fresh water input into the backwaters considerably decreases due

to low rainfall over the region. Hence a gradient of salinity develops from the mouth to the head of the backwaters

and thus the lower reaches behave as an extension of the Arabian Sea (Madhupratap 1987). Since the

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backwaters is geographically located in the tropical region, there is only minor seasonal variation of temperature

(Madhupratap 1987).

In the backwaters, phytoplankton biomass and production remains largely constant throughout the year,

although marked salinity variations arise seasonally as a result of heavy freshwater influx (Menon et al. 2000).

High river influx seems to have only minor effect on the overall phytoplankton production in the backwaters

(Qasim 2003). However, a qualitative shift in phytoplankton composition has been reported in the backwaters

during extremely low saline conditions (Menon et al. 2000). Among various size classes of phytoplankton in the

backwaters, nano-size fraction contributes majority of the primary standing stock and production all through the

year (Menon et al. 2000; Qasim 2003).

2.2. Sampling

The typical seasonal features in the hydrography of the study area was generated based on observations

from 20 stations in the backwaters, and 16 in the coastal waters during the summer monsoon (September 2004)

and pre-summer monsoon (April 2005). The idea behind the sampling was to differentiate the seasonal

hydrographic features in the backwaters and coastal waters (Figure 1). The sampling time was selected based on

the understanding that the pre-monsoonal and summer monsoonal hydrographical features would be fairly

reflected in observations during April and September respectively (Menon et al. 2000). In addition to the seasonal

measurements described above, from October 2004 to April 2005, monthly sampling was carried out at two

locations in the lower reaches of backwaters and coastal waters. This sampling was to present the gradual

environmental changes that occur in the backwaters and coastal waters from the summer monsoon to pre-

summer monsoon conditions (Figure 1).

The six rivers that empty into the backwater are responsible for the exceptionally high concentration of SiO4

(Sankaranarayanan and Qasim 1969). In contrast, non-point (local) sources have a major role in causing high

nitrate levels in the backwaters (Saraladevi et al. 1991). The industrial developments and intensification of

agriculture practices in the early 1970s have considerably accelerated the eutrophication in the backwaters

(Balachandran et al. 2001; Martin et al. 2008). The resultant increase in NO3 and PO4 concentration in the

backwaters is well reflected in the long-term data of NO3 and PO4 levels from the lower reaches of the Cochin

backwaters presented in Figure 2. The marked increase in NO3 and PO4 concentration began in the early 1970s

and attained elevated levels since 1980. It is important to see that the high nutrient levels prevailed in the

backwaters over several years have not caused any massive phytoplankton bloom or oxygen depletion within the

estuary so far, possibly due to adequate renewal of estuarine waters by the combined action of river discharge

and tidal exchange. Contrasting to NO3 and PO4, the salinity in the lower reaches of the backwaters do not show

any appreciable change over the years (Figure 2).

2.3. Methods

The surface temperature was measured using a centigrade thermometer. Salinity of the surface samples

were measured using a calibrated salinometer (Digi Auto 3G). During the monthly sampling, conductivity,

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temperature, depth (CTD) profiler recorded the vertical variation of temperature and salinity. From the CTD data,

the stratification of water column was decided in terms of ‘barrier layer’ which is the difference between isothermal

and isopycnal depths. In coastal areas where fresh water influx governs the stability of the water column, the

barrier layer thickness is a direct representation of the strength of the surface stratification (Balachandran et al.

2008a). In order to understand the transparency of the water column, a Secchi disc was operated in the coastal

and backwater locations during the monthly sampling. Attenuation coefficient of the water column during different

months was calculated from the Secchi disc data based on Pickard and Emery (1982).

Water samples were collected from surface (0.5m) and bottom using Niskin samplers. Samples for

dissolved oxygen (DO) were analysed by Winkler’s method. Nutrients (NO3, PO4, SiO4, and NH4) samples were

filtered through Whatman No.1 filter paper (pore size 1 µm) and analysed using a spectrophotometer (Shimazdu -

Japan) following standard procedures (Grasshoff et al. 1983). Water samples (500 ml) were filtered through

Whatman GF/F filter papers (pore size 0.7 µm) and the chlorophyll a was extracted using 90% acetone. The

measurements were carried out using a spectrophotometer following the procedure of Strickland and Parsons

(1972). Water samples (500 ml) were also collected for qualitative and quantitative analysis of phytoplankton and

preserved in 4% acid Lugol’s iodine. Water samples were concentrated to 10 ml following the settling and

siphoning procedure. 6-8 ml of the concentrated samples (6-8 replicates of 1ml each) was scanned in a Sedgwick

rafter counting chamber under an inverted epiflourescent microscope (Olympus IX 71) with 200-400X

magnification. The identification of phytoplankton was carried out based on standard literature (Subrahmanyan

1959; Tomas 1997). In the case of Trichodesmium, which formed a bloom in the coastal waters in April with an

areal extension of about 5 km, individual filaments were counted during the phytoplankton analyses (Figure 1). In

order to make a measure of the phytoplankton diversity in the backwaters and coastal waters, Shannon-Weaver

index (Shannon and Weaver 1963) was calculated using the species abundance data of the monthly sampling.

3. Results

3.1. Seasonal features in salinity and temperature

The seasonal variations of salinity and temperature in the study area are shown in Figure 3. During the

summer monsoon, freshwater was predominant in a major part of the backwaters (Figure 3) and as a result the

salinity in the barmouth area was also very low (3-5). In contrast, high salinity with less variability between

locations (av. 32 ± 1) was found in the coastal waters. During the summer monsoon period, the surface water was

warmer in the backwaters (26-34 °C) compared to the coastal waters (25.3 – 31 °C).

During the pre-summer monsoon, due to increased sea water incursion, the lower reaches of the

backwater behaved as an extension of the Arabian Sea with fairly high salinity (31- 33) (Figure 3). The low

freshwater influx was the main causative factor for the high salinity level (>31) in the backwater during the pre-

summer monsoon period. As usual, salinity was low (<1) at the upstream north of the backwater. The surface

temperature in the backwaters during the pre-summer monsoon period varied from 31 - 33.5 °C with relatively

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high values in the upstream region. As observed during the summer monsoon period, the surface temperature

was lesser in the coastal waters (29.8 - 30.5 °C) compared to the backwaters.

3.2. Monthly variations of salinity, temperature and transparency

The surface salinity in the backwaters gradually increased from near zero in October to 33 in April,

whereas in the coastal waters it increased from 31 in October to 33.7 in April (Figure 4a). By April, the lower

reaches of the backwaters showed prominent marine features with salinity >32 (Figure 4a &b). The warming of

surface waters from October to April was evident both in the backwaters and coastal waters. The backwater was

warmer throughout the study period compared to the coastal waters (Figure 4a&b). The stability (barrier layer) of

the water column in the backwaters and coastal waters increased from October to April (Figure 4a) and attained a

comparable level in April (Figure 4a). The attenuation coefficient of the water column in the backwaters was

markedly higher (lower transparency) in the backwaters from October to February compared to the coastal waters

(Figure 5). By March the water column transparency in the backwaters and coastal waters reached comparable

magnitude and in April both regions attained almost same amount of solar light availability in the subsurface

waters.

3.3. Seasonal features of DO and nutrients

During the summer monsoon period, DO concentration varied from 4.1- 7 mg L-1 with higher

concentration in the backwaters compared to the coastal waters (Figure 6a). During the period, all major nutrients

were found to be high (NO3 >2 μM; NH4 >0.5 μM, PO4 >1 μM and SiO4 >2.5 μM) both in the backwaters as well as

in the coastal waters (Figure 6 a – e). Among the sampling regions, the backwaters showed higher nutrient

concentration as compared to the coastal waters (Figure 6 b - e). Very high concentration of NO3 and silicate (>40

μM and > 90 μM respectively) was found in the upper reaches of the backwater during the period (Figure 6 b & e).

The DO and nutrient distribution during the pre-summer monsoon period is presented in Figure 6 f - j. DO

concentration varied spatially from 4 - 7 mg L-1 (Figure 6f). The NO3 and SiO4 level during the pre-monsoon period

(Figure 6 g & j) was markedly lower as compared to the summer monsoon period (Figure 6 b & e). In contrast, the

concentration of NH4 in the backwaters was higher during the pre-summer monsoon period than the summer

monsoon period (Figure 6c & h), with relatively high values in the upper estuary (21 - 35 µM). Similarly, the PO4

concentration in the backwaters was also higher during the pre-summer monsoon period (Figure 6d & i)

compared to the summer monsoon.

3.4. Monthly variations of DO and nutrients

The monthly variations of DO and nutrients in the backwaters and coastal waters are shown in Figure 7.

Except during October, DO level was consistently higher in the backwaters compared to the coastal waters. The

NO3 levels in the backwaters decreased initially from October to January and then increased towards April (18 µM

at the surface, and 14 µM at the bottom). In the coastal waters, the concentration of NO3 decreased considerably

from October (11 µM at the surface, and 28 µM at the bottom) to April (0.4 µM at the surface and 0.5 µM at the

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bottom). During most of the observations, especially during the pre-summer monsoon period, NO3 level in the

backwaters was higher than that of the coastal waters (Figure 7)

During the later part of the pre-monsoon period (March - April), the NH4 level was also markedly higher in

the backwaters compared to the coastal waters (Figure 7). Throughout the sampling period, PO4 and SiO4 were

higher in the backwaters compared to the coastal waters (Figure 7). SiO4 level in the backwaters and coastal

waters showed a gradual decrease from October to April with consistently lower values in the latter region

compared to the former.

3.5. Variations in chlorophyll a and phytoplankton

During both seasonal observations (September 2004 and March 2005), chlorophyll a was higher in the

backwaters compared to the coastal waters (Figure 8). Except in the southern part of the coastal region,

chlorophyll a was higher during the pre-summer monsoon period compared to summer monsoon. The

concentration of chlorophyll a was very high (>10 mg m-3) in the backwaters during most of the monthly

observations whereas it was relatively low (<8 mg m-3) in the coastal waters throughout the observations.

The phytoplankton community in the lower reaches of the backwaters and coastal waters were more or

less similar in composition (Table 2) and Nitzschia, Skeletonema, Thalassiosira and Thalassionema were the

dominant genera of diatoms in both regions. Trichodesmium was not recorded in the backwaters during the study,

whereas it was present in the coastal waters during January to April period. From October to March phytoplankton

abundance was high in the backwaters (av. 64500 ± 8000 No. L-1). In April, due to proliferation of

Trichodesmium, phytoplankton abundance in the coastal waters has increased significantly (186950 No. L-1). The

species diversity of phytoplankton was high in the backwaters in October, November and April (1.70, 1.72 and

1.79 respectively) whereas, it was high in the coastal waters in January, February and March (Figure 9).

4. Discussion

4.1. Anthropogenic influence and eutrophication

A significant change in the estuarine ecology due to human interference of the environment was reported

from the Hooghly estuary, at the head of the Bay of Bengal (Sinha et al. 1996; De et al. 1994). The above study

reported a considerable shift in phytoplankton composition including an elimination of Trichodesmium sp. in

recent decades. This was attributed primarily to the construction of Farakka Barrage on the River Ganga in April

1975. This barrage has brought about significant increase in freshwater discharge into the Hooghly estuary,

causing a major qualitative shift in the biological components (Sinha et al. 1996). However, such major decrease

in salinity has not been observed in backwater over the years (Figure 2). During the pre-summer monsoon, the

lower reaches of the estuary continues to have marine features and behave as an extension of the Arabian Sea

(Figure 3).

It is estimated that the backwaters is receiving 42.4 x 103 mol d-1 inorganic PO4 and 37.6 x 103 mol d-1 of

inorganic nitrogen through River Periyar, the major river associated with the backwaters (Naik 2000). Out of these

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nutrient inputs, there is an export 28.2 x 103 mol d-1 inorganic PO4 and 24 x 103 mol d-1 inorganic nitrogen into the

coastal waters which indicate the amount of the surplus inorganic nutrients available in the backwaters (Naik

2000). The long term data shows that NO3 and PO4 were in low levels up to early 1970s and since then it

increased due to augmented industrial and agriculture activities. During 1965, the surface PO4 and NO3 were 0.75

and 2.0 μM, which increased to 2.9 and 6 μM respectively by 2000. The overall trend shows a prominent increase

of NO3 and PO4 after 1975; and from 1980 onwards, the concentrations remained high (Balachandran et al.

2001). It is important to note that this comparison is based on available data from the lower reaches of the

backwaters as several researchers have sampled this region since 1965.

4.2. Seasonal changes in physical features in the backwaters and coastal waters

Normally, the surface layer stratification in marine waters is largely governed by solar heating (Pickard

and Emery 1982). However, in areas of high freshwater influx, the water column stability is governed primarily by

the upper layer of freshwater (Pickard and Emery 1982). This was found true during the present study also, since

the stability of the water column in terms of barrier layer thickness was high in the coastal waters (Figure 4).

Water column in the backwaters attained stability comparable to that of the coastal waters during April, when the

surface salinity in the former region was more or less same as that of the latter (Figure 6). Located in the tropical

region, Cochin backwaters receive the highest amount of solar radiation during the pre-monsoon period (626 g cal

cm-2 d-1) with 10-12 hours of sunshine (Qasim et al. 1968). However, monsoon associated heavy river runoff bring

high amount of suspended sediments into the backwaters which considerably reduce the transparency of the

water column having implications on the phytoplankton composition and physiology (Qasim et al. 1968). This

seasonal feature in solar radiation availability in the subsurface waters was well reflected in the Secchi disc data

collected during the present study showing higher attenuation coefficient, more prominent in the coastal waters,

during the monsoon period. As river runoff decreases by pre-monsoon period, the water column in the lower

reaches of the backwaters and coastal waters attains similar transparency level.

4.3. Seasonal changes in chemical parameters

Land drainage and river discharge during the summer monsoon brings in nutrient-enriched waters into

the backwaters (Saraladevi et al. 1983, 1986, 1991). As the rain and river flow decreases from October to April,

the nutrient input also decreases (Figures 5; also see Saraladevi et al. 1983). However, the PO4 levels in the

backwaters showed a steady increase from December to April but such changes were not very obvious in the

coastal waters. The observed increase in PO4 levels is believed to be the result of high salinity/pH combined with

tidal activity during the pre-summer monsoon which causes desorption of phosphate from the suspended particles

(Reddy and Sankaranarayanan 1972; Martin et al. 2008). It is important to note that concentrations of all nutrients

in the coastal waters (NO3 – 0.7 µM; PO4 – 0.5 µM; SiO4 - 0.9 µM, NH4 – 0.7 µM) were considerably lower than the

backwaters (NO3 – 8 µM; PO4 – 4 µM; SiO4 - 10 µM, NH4- 19 µM) during the pre-summer monsoon. The high

concentration of nitrogen compounds in the backwaters was due to the discharge of industrial, domestic and

agricultural wastes (Vijayan et al. 1976; Saraladevi et al. 1991; Qasim 2003).

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4.4. Phytoplankton composition and nutrient levels

It is usual that higher amount of phytoplankton stock occurs in the estuaries along the southwest coast of

India than the neighbouring coastal waters. This was primarily due to the surplus levels of nutrients available in

the backwaters throughout the year (Madhu et al. 2007; Balachandran et al. 2008b). This feature is found true

during the present study also, since none of the correlations between major nutrients and chlorophyll-a showed

significant positive relationship (Table 3).

The most common diatoms in the backwaters belongs to the genera Nitzschia, Skeletonema and

Thalassiosira, having high adaptability to survive in nutrient enriched estuarine conditions (Madhu et al. 2007).

The high abundance of Thalassiosira can also be considered as an indication of the deteriorated water quality

(Ramaiah et al., 1998; Raman and Prakash 1989). Similarly, Skeletonema dominate in areas where organic

waste inputs are high (Ramaiah et al. 1998). Prominent decrease in phytoplankton diversity observed in the

backwaters during January to March can be related with enriched levels of nutrients, which favors the proliferation

of a few species of diatoms (Ramaiah et al. 1998).

The diversity of phytoplankton in the backwaters and coastal waters were more or less comparable during

October to December, when nutrient concentrations were high in both regions. During January to March, the

phytoplankton diversity in the coastal waters has increased compared to the backwaters which may be linked to

the marked decrease in NO3 and SiO4 levels. The low NO3 and SiO4 levels in the coastal waters might have

decreased the ecological advantage of diatoms Skeletonema costatum and Nitzschia closterium, favoring the co-

occurrence of other diatom species in the environment. The environmental condition of high transparency and low

nutrients has also favored the proliferation of Trichodesmium in the coastal waters in April which in turn

decreased phytoplankton species diversity. In contrast, Trichodesmium was not encountered in the backwaters as

was the observation in the earlier studies (Alkershi 2002; Joseph 2005).

4.4. Impact of environmental factors on Trichodesmium

High solar radiation, warm and stable waters and low nutrients level are the favourable conditions for the

growth of Trichodesmium (Qasim 1970; Capone et al. 1998; Carpenter 1999). Recent modeling studies have

suggested that Trichodesmium distribution in marine waters is defined by high light intensity, stratified waters, and

low concentrations of dissolved inorganic nitrogen (Hood et al. 2001). The physiological response of

Trichodesmium to environmental features is difficult to measure, but efforts are progressing with laboratory

cultures elsewhere (Ohki et al. 1986; Lin et al. 1998; Stihl et al. 2001; Bell et al. 2005). Some of such studies

showed that Trichodesmium grows actively on a wide range of irradiances with optimal growth at 7 W m-2 (Ohki et

al. 1986). Similarly, Trichodesmium grows actively over a wide range of salinity (22–37), with optimum growth in

the range 30–37 (Bell et al. 2005; Hegde et al. 2008). Field studies from the coastal waters of Bay of Bengal and

Arabian Sea have also shown that the local species of Trichodesmium could form massive blooms with salinity

range of 29-31 (Jyothibabu et al. 2003).

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During the pre-summer monsoon, solar radiation in the Cochin backwaters and coastal waters is at its

seasonal highest with 10-12 hours of sunshine (Qasim et al. 1968). The salinity level in the backwaters and

coastal waters ranged between 33 - 33.5 which is well within the optimal salinity range (30 - 37) suggested for the

proliferation of Trichodesmium (Bell et al. 2005). The warm waters (>30°C) present in the study area was also

conducive for Trichodesmium growth (Capone et al. 1998; Hegde et al. 2008). Therefore, it is evident that salinity,

solar radiation and temperature present in the backwaters during the pre-summer monsoon period were

conducive for the growth of local species of Trichodesmium and therefore these environmental factors do not act

as limiting factors in the study area.

Recent studies have shown that Trichodesmium can assimilate compounds of nitrogen (NO3, NH4, amino

acids and dissolved organic nitrogen) from solutions. However, the normal growth and physiology of

Trichodesmium are inhibited by nutrients when present in high concentrations; presence of NO3 as low as 0.5 µM

is found to inhibits the growth of Trichodesmium and large initial concentration of NO3 (>10 µM) completely stops

the N2-fixation (Holl and Montoya 2005). Similarly, addition of NH4 to Trichodesmium cultures is found to inhibit

growth and nitrogen fixation (Lin et al., 1998). Some other studies showed that, high PO4 concentration also has a

strong inhibitory effect on the Trichodesmium growth (Ohki et al. 1986; Stihl et al. 2001). It is important here to

recall the fact that during the pre-summer monsoon period, Cochin backwaters have shown the presence of

exceptionally high levels of nutrients (NO3 – 8 µM; PO4 – 4 µM; SiO4 - 10 µM, NH4- 19 µM) than the coastal waters

(NO3 – 0.7 µM; PO4 – 0.5 µM; SiO4 -0.9 µM, NH4 – 0.7 µM). It is also to be noted that the disappearance of

Trichodesmium in the Cochin backwater coincides ever since (from mid 1970s) pronounced eutrophication has

been noticed. Therefore, we propose the exceptionally high levels of nutrients in the backwaters as the primary

cause for the absence of Trichodesmium in recent times. During the pre-summer monsoon, depleted nutrients

level in the coastal waters decrease the ecological advantage of a few species of diatoms over other

phytoplankton, favouring the proliferation of Trichodesmium (Devassy and Goes 1994). Certainly, more studies

would be needed to explore the extent of physiological impact of eutrophication on Trichodesmium in the

backwater. It is also important to verify the limiting effect of eutrophication on the occurrence of Trichodesmium,

proposed in this paper, in other similar estuarine systems along the Indian coast.

Conclusions

The environmental quality in the CBW and coastal Arabian Sea and its role on the differential occurrence

of Trichodesmium in respective regions during the pre-monsoon were analyzed. Long term data from the lower

reaches of the backwaters evidenced a five fold increase in NO3 and a six fold increase in PO4 levels after 1975.

Earlier studies on phytoplankton (before 1975) have shown the seasonal occurrence of Trichodesmium in the

lower reaches of backwater and coastal waters during the pre-summer monsoon. However, studies after 1975

haven’t encountered Trichodesmium in the backwaters, whereas, this species has frequently been reported from

the neighboring coastal waters during the pre-summer monsoon. While the physical features (salinity,

temperature, water column stability and transparency) in the backwaters and coastal waters were comparable,

the nutrient levels in the former region were 3 to 5 fold higher than the latter. Based on the current understanding,

11

it is proposed that high ambient level of nutrients in the Cochin backwaters is responsible for the absence of

Trichodesmium in recent times. High level of NO3 and PO4 in the backwaters possibly inhibit the normal growth of

Trichodesmium as observed in earlier experimental studies, whereas, its occurrence in the coastal Arabian Sea

was favoured with the depleted levels of nutrients.

Acknowledgements

The authors wish to thank Dr. S.R. Shetye, Director, National Institute of Oceanography for providing facilities.

The data used for this paper was collected associated with two research programmes of NIO Regional Centre,

Kochi, India; Viz, ‘Ecosystem Modeling of Cochin Backwaters’ (funded by ICMAM, PD- Chennai) and ‘Near-shore

Dynamics along the Kerala Coast with Special Reference to Upwelling and Mudbanks’ (funded by Centre for

Marine Living Resources and Ecology, Kochi). This is NIO contribution XXXXXX.

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16

Figure captions  

Figure 1‐ Station  locations  in the cochin backwaters and coastal waters;  ‘stars’ designate  locations of monthly sampling; Trichodesmium bloom observed in the coastal waters during April 2005 in the inset 

 Figure 2 ‐ Long – term variation in salinity, nitrate (NO3) and phosphate (PO4) in the lower reaches of CBW during the PSM period. Source: 1965 ‐ Sankaranarayanan and Qasim 1969; 1966 ‐ Qasim and Gopinathan 1969; 1968 ‐ Reddy  and  Sankaranarayanan 1972; 1970‐ Gopinathan 1972; 1972  ‐  Joseph 1974; 1973  ‐ Maniloth  and  Salih 1974;  1976‐  Lakshmanan  et  al.  1987;  1980  ‐  Nair  et  al.  1988;  1981‐  Saraladevi  et  al.  1986;  1982‐ Sankaranarayanan et al. 1986; 1984  ‐ NIO Data unpublished; 1986  ‐ Anirudhan 1988; 1989, 1992, 1993, 1995, 1996 ‐ NIO Data unpublished; 1997‐ Sheeba 2000; 2000 ‐Balachandran 2001; 2004 ‐ Martin et al. 2008; 2005 ‐  Present study.  Figure 3 ‐ Distribution of salinity and temperature during (a & b) September 2004 and (c & d) April 2005. The distribution plots are prepared similar to Balachandran et al. 2005, Pages 363‐364.  Figure 4  ‐  (a) Monthly  variation  in  salinity,  temperature, barrier  layer  and  (b)  vertical profiles of  salinity  and temperature at the two locations designated in Figure 1.  Figure  5‐  Monthly  changes  in  the  transparency  in  terms  of  attenuation  coefficient  at  the  two  locations designated in Figure 1  Figure 6 ‐ Distribution of DO and macronutrients during (a,b,c,d,e) September 2004  and (f,g,h,i,j) April 2005. The distribution plots  are prepared  similar  to Balachandran et  al. 2005, Pages 363‐364. The  concentration  in  the backwaters is shown as ranges whereas contouring by Surfer software is used for coastal waters.  Figure 7‐ Monthly variations in DO (mg L‐1) and macronutrients (µM) at the two locations designated in Figure 1  Figure 8‐ Distribution of chlorophyll‐a (mg m‐3) during (a) SM, (b) PSM and (c & d) monthly variation in chlorophyll a at the two locations designated in Figure 1 

 Figure 9‐ Monthly variations of phytoplankton abundance (No. L‐1) and (b) species diversity at the two locations designated in Figure 1 

 

 

 

 

 

 

 

 

17

Figure 1

1960 1970 1980 1990 2000 2010Years

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Figure 2

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Figure 3

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Figure 5

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Figure 9

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26

SL. No.

Phytoplankton genera

Hydrography Year (No. of sampling locations in brackets)

Source Salinity Temp

. (°C) NO3 (µM)

PO4 (µM)

1

(SM) Triceratium,Fragellaria, Coscinodiscus, Planktoniella (PM) Fragellaria, Coscinodiscus, Pleurosigma, Skeletonema (PSM) Skeletonema, Biddulphia, Coscinodiscus, Trichodesmium

1.0 14.6 32.3

27.7 28.2 31.5

- - -

- - -

1970 (2)

Gopinathan 1972

2

(SM) Spirogyra, Euastrum,Cosmarium, Closterium (PSM) Skeletonema, Prorocentrum,Ceratium, Trichodesmium

2.0 30.5

29.0 30.5

3.0 0.75

2.0 0.5

1972 (4)

Gopinathan et al. 1974

3

(SM) Skeletonema, Nitzschia, Coscinodiscus, Asterionella (PM) Skeletonema, Nitzschia, Coscinodiscus, Suriella (PSM) Skeletonema, Nitzschia, Coscinodiscus, Pleurosigma

1.0 6.0

30.5

27.5 29.5 30.0

- - -

- - -

1972 (3)

Kumaran and Rao 1975

4

(SM) Spirogyra, Euastrum,Cosmarium (PSM) Skeletonema, Prorocentrum,Ceratium, Dictyocha

0.5 30.2

28.0 29.3

1.6 0.6

0.3 0.4

1974 (6)

Gopinathan 1981

5

(SM) Fragellaria, Eucampia, Nitzschia, Coscinodiscus (PM) Eucampia, Coscinodiscus, Thalassiosira, Fragellaria (PSM) Oscillatoria, Skeletonema, Coscinodiscus, Microcystis

1.7 3.4

30.8

- - -

19.0 5.8 3.2

2.5 2.9 3.4

1981 (9)

Gopalakrishnan et al. 1988

6 (PSM) Peridinium, Oscillatoria, Pleurosigma, Navicula 30.0 31.5 - 2.8 1992 (2) Balasubramanian et al. 1995

7 (SM) Asterionella, Thalassiosira, Skeletonema, Nitzschia (PM) Thalassisothrix, Asterionella, Chaetoceros, Skeletonema (PSM) Thalassiosira, Chaetoceros, Rhizosolenia, Asterionella

34 31 33

26.0 30.0 31.0

5.89 1.44 2.92

2.7 0.9 1.1

2000 (1)

Alkershi 2002

*8

(SM) Aphanothece, Chroococcus, Dactylococcopsis, Gloeocapsa (PM) Chroococcus, Coelosphaerium, Coelosphaerium, Gloecapsa (PSM) Aphanothece, Chroococcus, Gloeocapsa, Synechococcus

5 27 30

28.6 29.0 31.2

12.0 7.0 6.0

5.0 1.5 2.0

2002 (8)

Joseph 2005

9

(SM) Nitzschia, Skeletonema, Synedera, Cocconeis (PM) Skeletonema, Coscinodiscus, Leptocylindrus, Nitzschia (PSM) Nitzschia, Skeletonema, Synedera, Thalassiosira

0.8 9.2

29.2

281 29.1 31.5

6.5 4.9

10.5

3.4 1.2 1.2

2003 (2)

Madhu et al. 2007

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(SM) Nitzschia, Skeletonema, Navicula, Leptocylindrus (PM) Thalassiosira, Skeletonema, Nitzschia, Navicula (PSM) Nitzschia, Skeletonema, Navicula, Thalassiosira

0 17.0 30.1

27.9 29.7 32.7

12.5 6.6 7.9

3.9 1.7 3.9

2005 (1)

Present study

Table 1 – Major genera of phytoplankton reported from the Cochin backwaters since 1970 (SM- Summer monsoon; PM- Post-summer monsoon; PSM- Pre-summer monsoon; – Not available; * study exclusively on cyanobacteria)

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October November December January February March April Phytoplankton CBW CMW CBW CMW CBW CMW CBW CMW CBW CMW CBW CMW CBW CMW Bacillariophyceae

Skeletonema costatum 11300 17520 19400 15600 17900 9600 16400 2600 14500 4560 16500 6560 23000 10900 Coscinodiscus sp. 130 340 150 60 420 410 250 50 210 600 180 60 1100 260 C. lineatus 440 200 200 - 50 - - 220 - 220 50 50 240 180 Leptocylindrus danicus 5200 450 4100 210 2500 100 1240 130 1950 530 360 260 2600 1300 Rhizosolenia sp. 120 40 60 300 40 120 240 30 60 150 120 120 60 130 R. alata - - - - - - - 200 - 120 - 160 - - R. imbricata - - - - - - - 400 - 60 - 120 - 160 Biddulphia sinensis 130 - 40 - - - - 50 240 150 60 250 - - Thalassionema nitzschioides 1600 200 800 1600 720 1300 2400 1350 1900 3500 2900 900 3000 1200 Thalassiosira sp. 4200 4500 15600 6500 18400 8000 7500 5560 11500 7560 5300 6400 12600 35560 Pleurosigma sp. 160 200 420 160 - 100 2600 120 240 200 190 200 320 320 Pleurosigma normani - 100 - - - 50 - 200 - 220 - 180 - - Navicula sp. 6420 4500 8300 4600 4500 13200 2400 12900 2600 11500 11300 11900 6400 12400 Nitzschia sigma 100 100 420 - 200 - - - 240 160 - - 230 230 N. closterium 16000 12000 21300 9400 15400 16700 32000 14000 29000 5000 24000 11000 21820 7820

Pyrrophyceae

Peridinium sp. 200 60 420 120 300 300 1200 1500 1080 - 1300 230 1300 260 Gonyaulax sp 40 50 240 40 160 100 360 130 450 230 600 430 160 - Gymnodinium sp - 100 - - - 40 - - - 120 - 100 - - Ornithocercus sp. 40 60 20 120 60 - 100 20 220 - - - - 20 Ceratium furca 20 70 100 100 40 50 - 60 180 60 240 120 620 400 C. lineatum - - - 50 - - - - - 120 - 100 - -

Cyanophyceae Trichodesmium erythraeum - - - - - - - 300 - 420 - 2100 - 112000

Others 4200 1100 2100 1300 720 600 2400 1000 1600 2400 3200 4200 1100 3600 Table 2- Monthly variation of dominant phytoplankton species (ind.L-1) in the CBW and CMW; (-) indicate absence. Trichodesmium filaments are counted during the study.

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Table 3- Correlation between chlorophyll a and macronutrients

Parameters R2 N Significance

Chlorophyll a Nitrate (NO3) 0.02 7 P >0.05

Chlorophyll a Phosphate (PO4) 0.29 7 P >0.05

Chlorophyll a Silicate (SiO4) 0.39 7 P >0.05

Chlorophyll a Ammonia (NH4) -0.5 7 P >0.05